1Field of the Invention
The present invention is directed to a nuclear magnetic resonance apparatus for the examination of first types of atomic nuclei that are nuclear-magnetically coupled to second types of atomic-nuclei.
2. Description of the Prior Art
The distribution of protons bonded in water molecules is the usual phenomenon investigated using a medical nuclear magnetic resonance apparatus, particularly a nuclear magnetic resonance tomography apparatus. These protons bonded in water molecules occur frequently in life forms and images having a good signal-to-noise ratio are obtained. For specific diagnostic inquiries, however, the examination of phosphorous (.sup.31 P) or carbon (.sup.13 C), for example, can provide more meaningful information. These atomic nuclei, however, are encountered in substantially lower concentrations in biological tissue. They are nuclear-magnetically coupled to the protons, so that the line width of resonant lines, for example, is substantially enlarged in spectroscopy, and thus the spectral resolution suffers. So-called decoupling pulses are therefore generally superimposed on the resonant frequency of the protons during the signal acquisition of the non-proton nuclei nuclear magnetic resonance signals. The technique thereby employed is described in the article by P. R. Luyten, "Broadband Proton Decoupling in Human .sup.31 P-NMR Spectroscopy" in NMR in Biomedicine, vol. 1, no. 4, 1989, pp. 177-183.
It is also known that a clear signal boost of the non-proton nuclei nuclear magnetic resonance signals can be achieved on the basis of the nuclear Overhauser effect by exciting protons before the excitation of the phosphorous atoms. This technique is described in the article by P. Bachert-Baumann, "In Vivo Nuclear Overhauser-effect in .sup.31 P-.sup.1 H Double Resonance Experiments in a 1.5-T Whole Body MR System" in Magnetic Resonance in Medicine 15, 1990, pp. 165-172.
Both in the decoupling and in the exploitation of the nuclear Overhauser effect, the examination subject must be charged not only with the resonant frequency of the type of atomic nuclei actually to be examined (for example .sup.31 P or .sup.13 C) but also must be charged with radio-frequency pulses on the proton resonant frequency. For this purpose, the conventional approach has been to equip a nuclear magnetic resonance apparatus with a second radio frequency channel (as explicitly recited, for example, in the second of the aforementioned references). This, however, involves substantial outlay.
U.S. Pat. No. 4,677,382 discloses the expansion of an NR apparatus that is initially suited only for one type of nucleus, by equipment so that a second type of nucleus can also be examined. To that end, two frequency converters are provided with which the transmission signal and the reception signal upon examination of the second type of nucleus are converted onto the Larmor frequency of the first type of nucleus by superimposition.